Network Working Group N. Harper
Internet-Draft Google Inc.
Updates: RFC8472 (if approved) October 22, 2018
Intended status: Standards Track
Expires: April 25, 2019
Token Binding for Transport Layer Security (TLS) Version 1.3 Connections
draft-ietf-tokbind-tls13-02
Abstract
Negotiation of the Token Binding protocol is only defined for
Transport Layer Security (TLS) versions 1.2 and earlier. Token
Binding users may wish to use it with TLS 1.3; this document defines
a backwards compatible way to negotiate Token Binding on TLS 1.3
connections.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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Internet-Draft TLS 1.3 Token Binding October 2018
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
Negotiating Token Binding using a TLS [RFC8446] extension as
described in [RFC8472] is fairly straightforward, but is restricted
to TLS 1.2 and earlier. Only one minor change is needed to use this
extension to negotiate Token Binding on connections using TLS 1.3 and
later. Instead of the server putting the "token_binding" extension
in the ServerHello like in TLS 1.2, in TLS 1.3 the server puts it in
EncryptedExtensions instead.
This document also non-normatively provides a clarification for the
definition of the TokenBinding.signature field from [RFC8471], since
TLS 1.3 defines an alternate (but API-compatible) exporter mechanism
to the one in [RFC5705] used in [RFC8471].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Token Binding TLS Extension
In TLS 1.3, the "token_binding" TLS extension may be present only in
ClientHello and EncryptedExtensions handshake messages. The format
of the "token_binding" TLS extension remains the same as defined in
[RFC8472].
A client puts the "token_binding" TLS extension in its ClientHello to
indicate its support for the Token Binding protocol. The client
should follow the same rules for when to send this extension and the
contents of its data as in section 2 of [RFC8472]. Since the
"token_binding" extension remains unchanged from TLS 1.2 to TLS 1.3
in the ClientHello, a client sending the "token_binding" extension in
a TLS 1.3 ClientHello is backwards compatible with a server that only
supports TLS 1.2.
A server puts the "token_binding" TLS extension in the
EncryptedExtensions message following its ServerHello to indicate
support for the Token Binding protocol and to select protocol version
and key parameters. The server includes the extension following the
same rules as section 3 of [RFC8472], with the following changes:
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Internet-Draft TLS 1.3 Token Binding October 2018
o The "token_binding" TLS extension is in EncryptedExtensions
instead of ServerHello.
o The server MUST NOT include both the "token_binding" extension and
the "early_data" extension on the same connection.
3. Interaction with 0-RTT Data
[RFC8446] requires that extensions define their interaction with
0-RTT. The "token_binding" extension MUST NOT be used with 0-RTT
unless otherwise specified in another draft. A client MAY include
both "early_data" and "token_binding" extensions in its ClientHello -
this indicates that the client is willing to resume a connection and
send early data (without Token Binding), or negotiate Token Binding
on the connection and have early data rejected.
4. Clarification of TokenBinding.signature
This non-normative section provides a clarification on the definition
of the TokenBinding.signature field when used on a TLS 1.3
connection.
The Token Binding protocol [RFC8471] defines the
TokenBinding.signature field in terms of an exported keying material
(EKM) value as defined in [RFC5705]. TLS 1.3 [RFC8446] provides an
equivalent interface in section 7.5. For clarity, using the
terminology from [RFC8446], the EKM used in section 3.3 of [RFC8471]
in TLS 1.3 is the exporter value (section 7.5 of [RFC8446]) computed
with the following parameters:
o Secret: exporter_master_secret.
o label: The ASCII string "EXPORTER-Token-Binding" with no
terminating NUL.
o context_value: No context value is supplied.
o key_length: 32 bytes.
These are the same input values as specified in section 3.3 of
[RFC8471].
5. Security Considerations
The consideration regarding downgrade attacks in [RFC8472] still
apply here: The parameters negotiated in the "token_binding"
extension are protected by the TLS handshake. An active network
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